This document shows how to use a mutex and semaphores in order to synchronize two tasks in a FreeRTOS and SDK project. For this. This tutorial shows how to use FreeRTOS mutexes to avoid race conditions between different threads. We will show how preemption could. #include “FreeRTOS.h” #include “semphr.h” SemaphoreHandle_t xSemaphoreCreateMutex(void);. Summary. Creates a mutex type semaphore, and returns a.

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They include a priority inheritance mechanism which only makes sense if the mutex is given and taken from a task, not an interrupt. For this example it is used SDK 2. The data size is 0 as we don’t want to actually store any data – we just want to know if the mutexx is empty or full.

The queue length is 1 as this is a binary semaphore. The archive is updated every week, so freettos not always contain the very latest posts. Mutexes use the same semaphore access API functions so also permit a block time to be specified. Sign up or log in Sign up using Google. The synchronization is based on bilateral rendezvous pattern. The mutex must have previously been created using treertos call to xSemaphoreCreateRecursiveMutex. After install SDK 2. This makes binary semaphores the better choice for implementing synchronisation between tasks or between tasks and an interruptand mutexes the better choice for implementing simple mutual exclusion.

Unlike binary semaphores however – mutexes employ priority inheritance. It just minimises its effect in some situations.

In this case it is desirable for the initial count value to be equal to the maximum count value, indicating that all resources are free. If only one task is changing the variable, and all other tasks just one in your case is reading the variable, then you don’t need to protect the variable at all provided the variable can be written in one go.


Hard real time applications should be designed such that priority inversion does not happen in the first place. This macro must not be used on mutexes created using xSemaphoreCreateMutex. If they used it in the same time, there would be conflicts, so a mutex is used to synchronize the two tasks. The mutex must have previously been created using a call to xSemaphoreCreateRecursiveMutex. This is only true when there is only one writer – if more than one task was writing to the variable then it would need protecting.

Return the handle of the task that holds the mutex specified by the function parameter, if any.

Semaphore / Mutexes

A mutex used recursively can be ‘taken’ repeatedly by the owner. The time in ticks to wait for the semaphore to become available. Mutexes – The priority of a task that holds a mutex will be raised if another task of higher priority attempts to obtain the same mutex.

The count value is therefore the difference between the number of events that have occurred and the number that have been processed. The xSemaphoreTake and xSemaphoreGive macros should not be used. Whereas binary semaphores are the better choice for implementing synchronisation between tasks or between tasks and an interruptmutexes are the better choice for implementing simple mutual exclusion hence ‘MUT’ual ‘EX’clusion.

Sign up using Facebook. Fail Safe File System: This is the handle returned when the semaphore was created. Introduction Binary semaphores and mutexes are very similar but have some subtle differences: Posted by rtel on March 13, Mutexes created using this macro can be accessed using the xSemaphoreTake and xSemaphoreGive macros.

The maximum count value that can be reached. The API documentation page for that function has a small example. Posted by blavo1 on March 13, In the example I am working on: Binary Semaphores – A binary semaphore used for synchronization does not need to be ‘given’ back after it has been successfully ‘taken’ obtained.


Do not delete a semaphore that has ferertos blocked on it tasks that are in the Blocked state waiting for the semaphore to become available.

Writing a Mutex example code The mutex example code is used to demonstrate how to use a mutex to synchronize two tasks. We use cookies to provide and improve our services. Stack Overflow works best with JavaScript enabled.

When the semaphore reaches this value it can no longer be ‘given’. A mutex provides mutual exclusion among tasks, when they access a shared resource. Macro to obtain a semaphore. Task1 must change a variable called frequency periodically and task2 will blink led each time with the new frequency.

[RTOS Support] simple Mutex code example example

According with this proposal it is necessary to follow the below mmutex. The semaphore was created successfully. Macro to recursively release, or ‘give’, a mutex type semaphore.

Mutexes include a priority inheritance mechanism, binary semaphores do not. Macro to recursively release, or ‘give’, a mutex type semaphore.

Mutexes are given and taken using xSemaphoreGive and xSemaphoreTake [except recursive mutexes]. This API creates a mutex, and returns a handle by which the created mutex can mutrx referenced. This means that if a high priority task blocks while attempting to obtain a mutex token that is currently held by a lower priority task, then the priority of the task holding the token is temporarily raised to that of the blocking task.

Recursive mutexes cannot be used in interrupt service routines. Below is a static menu.